1. Field of the Invention
The present invention relates to rotation sensors. More particularly, this invention pertains to an improved ring laser gyroscope.
2. Description of the Prior Art
The ring laser gyroscope is a rotation sensor that senses rotation about an axis that is perpendicular to the plane of a cavity formed within a frame, preferably of glass ceramic or other low thermal coefficient material. Beams of laser light circulate in opposite directions within the cavity. In accordance with the well-known Sagnac effect, the frequencies of the two beams are altered in opposite senses (that is, one is increased while the other is decreased) by rotation about the axis and the beat frequency between the two beams then provides a measure of rotation.
Lasing is effected within the cavity by the interaction of photons with an excited medium which acts as an amplifier. In a d.c. configuration, the medium is excited by the interaction of a fill gas, typically HeNe, with flows of electrical current between electrodes arranged about or within the gyro frame. Alternatively, in an r.f. actuated device, the medium is excited by means of an electromagnetic field that oscillates at radio frequencies. Only two counterrotating lasing modes need to be supported within the ring laser cavity to obtain a measure of rotation. In a planar cavity, the counterrotating beams are linearly polarized whereas a nonplanar cavity can support both right and left circularly-polarized modes.
The (gain v. frequency) profile of an excited gain medium formed within a cavity is subject to a line broadening effect that reflects the Doppler shifts that occur as a result of the gas atoms moving in the gain medium. As a consequence, the width of the "gain curve" is considerably broader than the natural line width of the Ne transition that is responsible for the 6328 A line and exhibits a bell shape as a result of the Gaussian distribution of the velocities of the gas atoms.
The modes that will resonate, or lase, within a cavity in the presence of a sustaining gain medium are determined by the cavity's geometry. In general, a planar cavity with dielectric mirrors will support only linearly polarized modes while a nonplanar cavity can sustain circularly polarized modes. In the case of a nonplanar cavity, the right and left circularly polarized modes are separated in frequency, an effect known as "reciprocal splitting".
The multioscillator is a type of ring laser gyroscope that is characterized in operation by four lasing modes arranged into two counterpropagating pairs. One of these pairs comprises left circularly polarized light while the other comprises right circularly polarized light. In such a device, the two polarizations are separated in frequency by the reciprocal splitting effect of the (generally non-planar) ring cavity. The frequencies of the clockwise and counterclockwise (or "anti-clockwise") modes are separated in frequency by a nominal amount known as "nonreciprocal splitting". When the device is rotated, such rotation is reflected in equal and opposite changes in the amounts of nonreciprocal splitting between the frequencies of the counterpropagating beam pairs of the right and left circularly polarized modes.
The multioscillator permits one to solve the critical "lock-in" problem by using a d.c. biasing technique without the normal intolerable bias sensitivities associated with drift in the magnitude of the bias. This is accomplished by operating two rotation sensing counterpropagating beam pairs in the cavity simultaneously which are distinguished by their modes of polarization. The applied bias is configured to be equal but opposite between the pairs so that the summed rotational output is independent of any drift. Further, the presence of two frequency differences for deriving rotation provides the user with double the scale factor, and, hence, twice the sensitivity, of a two mode gyroscope. Finally, as is well known in the art, random walk is significantly reduced in the multioscillator, enhancing the reliability of the measurement that is provided by such an instrument.
While a non-planar cavity characteristically supports both left and right circularly polarized modes, the non-reciprocal splitting between counterpropagating beams of the same polarization is conventionally obtained by introducing a Faraday element. The addition of such an element poses a number of drawbacks. These include the fact that they are lossy. Further, the addition of a Faraday element involves costly processes and the resulting device is hampered in regard to nuclear hardening considerations. Commonly-owned pending U.S. patent application Ser. No. 928,069 of Graham John Martin for "Geometrically Biased Ring Laser Gyroscope" teaches the imposition of a magnetic field on the gain medium to suppress two of the four modes that normally lase in a multioscillator-type configuration. The remaining two modes propagate in opposite directions around the cavity and, thus, can provide rotational information. In addition, since the two modes are of orthogonal polarizations they are separated in frequency by an amount that is determined by cavity nonplanarity. Hence, such a configuration effectively achieves nonreciprocal splitting without requiring an intracavity element.
The type of two-mode geometrically biased device described in that patent application has been found to be inadequate for accurate ring laser gyroscope use as a consequence of instabilities in the reciprocal splitting. Such instabilities are related to effects in the dielectric mirrors of the cavity. The reduction from four to two modes in the multioscillator removes the advantageous common mode rejection. This shortcoming is addressed in commonly-owned U.S. patent application Ser. No. 115,018 of Graham John Martin entitled "Split Gain Multimode Ring Laser Gyroscope and Method" that again utilizes a magnetic field applied to the gain medium to suppress certain modes from lasing. In contrast to the former device, four-mode operation is restored by operating across the cavity resonances from two different longitudinal mode sets. In this manner instabilities in the geometrically-induced reciprocal splitting are satisfactorily moved by the common mode rejection provided by the split-gain four-mode configuration. However, at the same time, the latter device removes lock-in without the use of intracavity elements.
While the above patent application teaches a device that does not require the introduction of an undesired intracavity element for operation, the device disclosed in the patent application, like other laser rotation sensors, is subject to operational problems that derive from "sharing" of the excited gain medium by multiple cavity modes. As discussed above, the gain curve characteristic of an excited gain region is subject to a considerable line broadening effect. In a realistic HeNe-filled ring cavity, for example, the cavity mode spacings are generally less than the width of the gain curve.
The sharing of the gain medium introduces harmful mode competition effects. Such effects result from the need for the different modes to "compete" for gain from the same set of gain medium atoms (Ne atoms in the case of a HeNe-filled ring laser). In fact, such effects have limited current practical ring laser gyroscopes to utilizing lasing transitions in atomic neon. In general, mode competition effects prevent the stable operation of two simultaneously-counterpropagating modes; a mix of the Ne.sup.20 and Ne.sup.22 isotopes has the fortunate property of allowing sustained lasing with counterpropagating beams. The mode competition effects also cause undesired interactions between the mode frequencies known as "mode pulling" and "mode pushing". Such effects upon the frequencies of the modes are quite disruptive of sensor operation which depends upon rotation-induced changes in the frequencies of such modes for determining the value of angular rate.